Trump announced the action against Facebook, Twitter, and Google's YouTube, along with their respective CEOs Mark Zuckerberg, Jack Dorsey, and Sundar Pichai, and demanded that his accounts be reinstated. Under Section 230 of the 1996 Communications Decency Act, social media platforms are allowed to moderate and remove content that violate standards, so long as they are acting in "good faith". Immediately after Trump's announcement he started fundraising to raise money to fund the lawsuits.
Trump has been suspended from the platforms since January, when his followers violently stormed the Capitol, trying to block Congress from certifying Joe Biden's win. The companies cited concerns that Trump would incite further violence and have kept him locked out. Link
